Typically, the masts of sailing vessels are supported in a generally vertical position by several shrouds or stays which are connected to the deck. These stays are made of metal, fiber, or plastic rod, or of cables made of strands of wire. In order to rig a vessel properly, these stays should be placed in a predetermined tension. Shrouds or stays on smaller sailing vessels are frequently tensioned in an empirical manner when a vessel is rerigged or tuned, or when an individual stay is replaced. The person making the replacement or adjustment may resort to feeling the stay's give or movement to gauge whether it is tight or loose enough, or to match the tension of an opposing stay. As this is a very subjective method, the results are not always optimal. It is often necessary to resort to a back-and-forth method of adjusting one stay, then its opposite, then returning to the first, then returning to the second, again and again until what is felt to be the optimum tension is achieved. This subjective method can result in unequal loading on the mast. This can also result in a mast which is too tightly or too loosely held in position, and thus fail to optimize the sailing characteristics of the vessel. In addition, if such an operation is not carried out with great care, the stay can be stressed beyond its elastic limit, and will then not return to its former length when relaxed. In addition, it may be susceptible to breakage. Such overstressing can damage mounting and termination fittings.
Tensiometers and the like have long been used to measure the tension of sailboat shrouds and stays. Many such devices measure the effort required to deflect a stay under tension a predetermined amount, or measure the extent of deflection caused by a predetermined eccentric load applied to a tensioned stay.
Other devices such as electrical strain gauges are placed in line with shrouds or stays. The variable resistance is measured and displayed on appropriate electronic units.
Certain devices measure the tension in a cable or rod as a function of elongation. It is well known that elastic elements such as sailboat rigging stays will elongate according to the formula EQU .DELTA.L=LT/EA
where
.DELTA.L=Elongation of tensioned element PA1 T=Tension PA1 L=Length PA1 E=Elastic modulus PA1 A=Cross-sectional area of material.
By transposition, we can obtain EQU L=EA.DELTA.L/ T.
Values for T based upon convenient percentages of breaking strength will, in combination with unitary, convenient values for .DELTA.L, determine a single value for L for a given tensioned element.
By way of example, consider 3/8" 1.times.19 stainless steel wire rope, manufactured with 19 wires spirally wrapped around a single straight core wire. Such wire rope has a nominal breaking strength of 17,500 pounds, a cross-sectional area of 0.088979 square inches, and an elastic modulus of 22.times.10.sup.7. A first point of measurement on a length of such wire rope 69,906" from a second point of measurement when released of all tension will be found to have moved 1/32" away from the second point for each 875 pounds of tension applied along the longitudinal axis of such rope. This 875 pounds is equal to 5% of the breaking strength of this type and size of wire rope. Thus, by constructing indicia with graduations equal to 1/32", elongations caused by a loading equivalent to 5%, 10%, etc., of breaking strength of such rope can be directly indicated if the base line of the indicia is located 69.906" from a first point of measurement. By locating these indicia a suitable distance from the fixed point of connection with the stay, the elongation of the stay can be measured and the tension generated therein directly determined.
It is therefore a general purpose of the present invention to provide an improved gauge which is simple, easily applied, and inexpensive, suited for use in measuring tension as a function of elongation of sailboat rigging stays.